The present invention is directed to microwave phase shifters and more particularly to latching microwave ferrite phase shifters and methods for more quickly switching them between desired phase shifting states.
The phenomenon accompanying the arrangement of two oppositely and equally magnetized ferrite slabs, separated by a di-electric slab and located within a microwave waveguide are well known. Reversal of the magnetization in both ferrite slabs causes a change in insertion phase for a given direction of propagation. This change in insertion phase constitutes a phase shift for that direction of propagation. Further, a change in the magnitude of the magnetization of the two ferrite slabs produces a change in insertion phase for a given direction of propagation. It is therefore desirable to quickly and precisely adjust the magnetization of the ferrite slabs in order to control the phase shift and/or the insertion phase of the waveguide. The present invention is for a method of setting the magnetization levels of the ferrite slabs which is faster than prior art methods and is for an apparatus for executing the method which is simplified in construction over prior art apparatus.
The theory underlying the phenomenon of the ferrite phase shifter need not be fully understood in order to appreciate the present invention's contribution to the art. However, a brief explanation follows in order to provide some background. The reader desiring a more comprehensive treatment is referred to an article entitled "Non-reciprocal Remanence Phase Shifters in Rectangular Waveguide", IEEE MIT-2, February 1967 by W. J. Ince
The theory underlining the phenomenon of the ferrite phase shifters states that the insertion phase of the structure varies with the magnetization of the ferrite slab as shown in FIG. 1. Thus, for a given direction of propagation, a ferrite slab magnetized counterclockwise to some saturated state represented by Point 1 would exhibit an insertion phase .0..sub.1. If the magnetization were switched to a clockwise saturated state represented by Point 2, the device would exhibit an insertion phase .0..sub.2. The phase shift experienced in going from state 1 to state 2 would then be .0..sub.2 -.0..sub.1.
On the other hand, if the magnetization were changed in magnitude, for example by going from Point 1 to a Point 3, the phase shift would be .0..sub.3 -.0..sub.1. Therefore, a continuum of phase states are available between the limits of .0..sub.1 and .0..sub.2, which represent saturated states of magnetization. In FIG. 1 the point of zero magnetization is represented by an insertion phase .0..sub.0.
The state represented by point 1 in FIG. 1 is called the "long state" since the insertion phase is highest and that represented by point 2 is called the "short state". The two branches of the curve of FIG. 1 are referred to as clockwise (CW) and counterclockwise (CCW) referenced to the direction of propagation of the wave. Thus, if the ferrite were magnetized in a particular direction (CCW) representing state 1 in FIG. 1 for the transmit direction of propagation, it would exhibit a phase .0..sub.1 to the transmit signal. For the received signal, however, the phase would be .0..sub.2 if the ferrite remains in the same switched state as for the transmit direction of propagation. If the ferrite were switched to the oppositely magnetized state, the received signal would experience an insertion phase .0..sub.1, exactly the same as for the earlier transmitted signal, since the opposite state of magnetization is now counterclockwise referenced to the received signal direction. It is a fundamental principle that reversal of the magnetization, without changing its magnitude, throughout an electromagnetic system has exactly the same effect for a fixed direction of propagation as reversing the direction of propagation without changing the magnetization.
The requirement that the receive and transmit phases be virtually identical requires that the direction of magnetization be reversed without any changes in magnitude of the magnetization no matter what the value of the magnetization may have been.
To make the device a latching phase shifter, one in which no energy is required to hold the device in a given state, it is necessary that the magnetic path be closed into a continuous loop, such that a "square" hysteresis loop is obtained as shown in FIG. 2. To switch the magnetization from a remanent of -4.pi.M.sub.r to the opposite remanent value of +4.pi.M.sub.r, a current pulse is applied to provide a positive magnetization field (H). The magnetization (M) moves upward to the point +4.pi.M.sub.max, at which time the current is turned off reducing the H field to zero. The magnetization then "falls back" to a remanent value of +4.pi.M.sub.r and remains there without a holding field, i.e. it
is "latched". To return to the -4.pi.M.sub.r remanent value, an opposite H field is applied and the magnetization moves to -4.pi.M.sub.max and then to -4.pi.M.sub.r when the H field is reduced to zero. It is important to note that the alegebraic signs plus and minus used in FIG. 2 refer to a particular physical direction of magnetization whereas the notations counterclockwise and clockwise in FIG. 1 are referenced to the direction of propagation. Therefore, if minus in FIG. 2 corresponds to counterclockwise in FIG. 1 for the transmit case, it follows that plus in FIG. 2 is counterclockwise for the received case. The actual dynamics of fast-switching cannot be represented by a hysteresis loop as shown in FIG. 2, which is representative of quasi-static changes. Nevertheless, the concept does represent rest conditions and is very useful in understanding the device.
It is also known in the prior art to drive the ferrite slab to partially saturated states intermediate of the fully saturated states of 4.pi.M.sub.r and -4.pi.M.sub.r by use of a two step sequential switching technique in which a fully saturated state (reference) is followed by a partially saturated state. Referring to FIG. 2, suppose the ferrite slab is at a partially saturated value of -4.pi.M.sub.a for a transmit mode, corresponding to a phase state .0..sub.a for the transmit mode. To set the ferrite slab to a receive mode in the same state, a large forward pulse creating a large H field is applied to move the magnetization to +4.pi.M.sub.max. The pulse is then removed to allow the magnetization to fall to the +4.pi.M.sub.r state. This is called a reset operation since the magnetization of the ferrite slab is reset to a known reference value. Shortly thereafter, within one microsecond, a smaller controlled negative pulse creating a negative H field is applied to drive the magnetization to Point A and then removed to allow the magnetization to fall back to the new remanent value of +4.pi.M.sub.a. This is called a set operation. Control of the set operation is crucial in that the whole concept of switching rests on the ability to switch a controlled amount of magnetic flux in a repeatable fashion over some environmental range. After the reset and set operations are completed the phase shifter is in phase state .0..sub.a for the receive mode the same as the phase state for the transmit mode. Thus, a two-step operation, reset and set, is required to place the ferrite slab in the appropriate magnetization state. According to the prior art, by using such a two pulse operation, the phase state can be changed from any value to any other value.
The use of minor remanent loops intermediate of positive and negative saturation is well known. See, for example, U.S. Pat. Nos. 3,524,152 to J. P. Agrios et al and 3,340,484 to W. W. Siekanowicz et al. Patents discussing the use of remanent states intermediate of positive and negative saturation typically describe the well known prior art switching technique of first driving the set toroid to a reset condition and then driving the newly reset toroid to the desired set condition as discussed above. See, for example, U.S. Pat. Nos. 4,042,831 to Lenhoff, Jr. and 3,988,686 to Beall et al.
It has also been recognized in the prior art that the degree of phase shift may be varied from a maximum phase to a minimum phase by maintaining the flux in one ferrite toroid constant and switching the flux in the other ferrite toroid as illustrated by U.S. Pat. No. 3,681,715 to Freibergs. This patent also discloses using independent means for energizing each of the ferrite toroids.
The controlled flux technique previously discussed has definite advantages in terms of ease of assembly and microwave performance parameters. However, the switching technique used for a device utilizing the controlled flux technique involves two distinct switching operations (the reset and set operations) which are accomplished sequentially in time. In addition to the two switching operations, it is necessary to insert an additional delay after the reset pulse to allow sufficient time for the ferrite element and the electronics to settle before the set pulse is applied. Using a technique of this type, a recent X-band phase shifter produced at Electromagnetic Sciences was switched in twelve microseconds using a controlled flux driver to produce a multitude of desired phase states. However, in a phased array radar, using non-reciprocal phase shifters, if a pulse of rf energy is sent out by a transmitter, the ferrite elements in the phase shifter must be switched between the end of the transmit pulse and the beginning of the receive pulse reflected from the target. For this reason, the time necessary to switch the device from the transmit state to the receive state is a very critical parameter governing the minimum range target which can be observed. To observe a target at a distance of five hundred feet a switching time of approximately one microsecond is required. It is obvious that the prior art controlled flux switching technique does not provide suitable performance in certain applications such as phased array radars with such switching times.
Another prior art technique for controlling the magnetization states of the ferrite slabs is the mulit-bit technique. For example, a two-state phase shifter can be made by simply switching between -4.pi.M.sub.r and +4.pi.M.sub.r shown in FIG. 2. This corresponds, for example, to a switching between .0..sub.1 and .0..sub.2 in FIG. 1. If the phase shifter is in state .0..sub.1 at remanent value -4.pi.M.sub.r for the transmit mode, it can be put in the same state .0..sub.1 for the receive mode by switching the ferrite slab to the +4.pi.M.sub.r remanent value. A four-bit digital phase shifter can be constructed by placing four distinct pieces of ferrite in tandem, with the length of each piece twice as long as the preceding one and adjusting them to have 221/2.degree., 45.degree., 90.degree. and 180.degree. of phase shift, respectively. Each ferrite piece has it own pair of latch wires and it own driver. The bits are separated by dielectric spacers to avoid interaction of latch currents with adjacent ferrites. Such a configuration has been useful in the past in fast-switching situations when the switching time of the controlled flux phase shifter was excessive. A two microsecond switching time which can be provided by the multi-bit switching technique is unobtainable using the previously discussed prior art controlled flux phase shifters.
Although the use of multi-bit phase shifters provides considerably faster switching times, in situations where more than four bits are required the smaller bits become too small to handle and the final assembly becomes cumbersome. The many interfaces required to prevent interactions because of the accuracy specifications of different materials produce reflections that preclude achievement of various design specifications. Further, the driver needed to drive each of the ferrite pieces becomes extremely expensive. The many latch wires and interfaces tend to produce higher order modes that impair phase accuracy. Thus, although the multi-bit phase shifter provides desirable switching times, in many applications it cannot be used because of the cumbersome physical design.